1. Field of the Invention
The present invention relates to a method and an apparatus for binding pollutants in flue gas in a flue gas channel and furnace of one or more combustion plants, e.g., a fluidized bed boiler and/or a suspension-fired boiler.
One of the one or more combustion plants discussed above typically comprises a furnace, a flue gas channel connected to the furnace, a contact reactor provided in the flue gas channel, a dust separator provided in the flue gas channel after the contact reactor, and an ash recirculation conduit connecting the dust separator and the contact reactor, the recirculation conduit having a moistening device arranged therein. The method may include the following steps:
(a) maintaining reactions resulting in the production of flue gas containing pollutants in the furnace; PA0 (b) introducing limestone or other calcium oxide forming material into the furnace for binding pollutants in the flue gas in the furnace; PA0 (c) causing flue gas to flow through the flue gas channel to the dust separator, wherein ash containing unreacted calcium oxide is separated from the flue gas; PA0 (d) conducting a portion of the ash separated from the flue gas to the moistening device, wherein water amounting up to 50% of the weight of the ash is mixed into the ash; and PA0 (e) mixing the ash moistened in step (d) into the flue gas flowing in the contact reactor, whereby ash is entrained with the flue gas to the dust separator, thus binding pollutants in the flue gas. PA0 (f) causing a portion of the ash moistened in step (d) and mixed into the flue gas to fall down to the retention tank for ash; PA0 (g) hydrating at least a portion of the calcium oxide in the ash to form calcium hydroxide in the retention tank; and PA0 (h) recycling at least a portion of the ash from the retention tank to at least one of the one or more combustion plants.
2. Related Background
The combustion of sulfurous fuels in the furnace of a boiler generates sulfur dioxide (SO.sub.2). The sulfur content of the flue gases emanating from the furnace can be reduced by wet, half-dry and dry desulfurization methods. By these methods, flue gases are allowed to react with a sulfur-binding compound which is in a liquid, wet (but drying during the process) or an initially dry form.
The sulfur content of flue gases can also be reduced by introducing directly to the furnace a sulfur-binding compound, e.g., calcium hydroxide (Ca(OH).sub.2), or a material, such as limestone or dolomite, which forms a sulfur-binding compound, e.g., calcium oxide (CaO). The last-mentioned method is efficient, especially in fluidized bed boilers, since the retention time of a sulfur-binding compound in the furnace can be quite long.
In a fluidized bed, calcium carbonate (CaCO.sub.3) in limestone or dolomite calcines to form calcium oxide (CaO) which binds sulfur dioxide and forms calcium sulfate (CaSO.sub.4) and calcium sulfite (CaSO.sub.3). The problem in this process is a dense layer of calcium sulfate formed on the surface of the calcium oxide particles reacting with sulfur dioxide, which layer prevents the calcium oxide inside the particles from participating in the reaction. Therefore, in order to reach a sulfur binding level of over 90%, the mole ratio of calcium to sulfur in the material supplied to the bed, i.e., the Ca/S-ratio, has to be as high as 3-5 or even higher in some process conditions.
The calcination of lime requires energy, whereby the introduction of a large amount of lime into the furnace reduces the boiler efficiency. A high lime content in the fluidized bed also increases the amount of NO.sub.x emissions from the boiler. Further, there is a risk of deposits forming on the back pass, when a lot of lime is introduced into the furnace.
A high sorbent consumption adds to the costs, and the amount of solid waste produced by the plant is increased as well. Further, the lime in the ash has to be slaked (i.e., combined chemically with water) before storing or heaping, if the ash contains a significant amount of burnt lime.
The utilization rate of the sulfur-binding sorbent in a combustion plant can be improved by recovering unreacted calcium oxide in the bottom ash or fly ash and feeding it to the flue gas or back to the furnace after activation. The activation of the sorbent can be effected by extracting the calcium sulfate layer from the surface of the calcium oxide particles and slaking the lime, i.e., by hydrating the calcium oxide (CaO) so as to obtain porous calcium hydroxide (Ca(OH).sub.2).
U.S. Pat. No. 4,185,080 discloses a multi-stage method, in which calcium oxide is separated from the fluidized bed or fly ash, calcium sulfate is removed and calcium oxide is slaked by a stoichiometric quantity of water in a separate slaking apparatus producing dry, extremely fine-grained calcium hydroxide which is recycled to the combustion zone or to the gases emanating from the combustion zone.
U.S. Pat. No. 4,309,393 discloses another method based on the use of a separate hydrating apparatus, in which the unreacted calcium oxide of the bottom ash is hydrated by a large amount of water and supplied to the flue gases as a wet suspension.
U.S. Pat. No. 4,279,873 discloses a method, in which the flue gas is allowed to react in a spray drier with an aqueous suspension consisting of fly ash and slaked lime produced in a separate hydrating apparatus. The water content of the suspension is adjusted to obtain a temperature of the flue gas, which is 8-40.degree. C. above the saturation temperature. It has been found that in these conditions, the sulfur oxides are bound most efficiently by calcium hydroxide. This patent also discloses the recycling of dry powder, collected from the bottom of the spray drier, to the suspension.
The published patent application WO 96/16722 discloses a method of removing sulfur dioxide from flue gas, in which fly ash is recycled, via a dust separator and a moistening device in the flue gas channel, together with fresh calcium oxide to be added during the recirculation. Water is mixed into the ash in the moistening device and the mixture is allowed to recirculate in the system as many times as is needed for the calcium oxide to have time to get hydrated completely. Therefore, no separate hydrating device is used in this method. When applying the method, the particles are typically in a moistened state only for 10-20 seconds during each cycle, whereby the number of cycles has to be high enough in order to accomplish sufficient hydration. Thus, the particle content of the flue gas is high, which causes erosion. Further, the size of the dust separator of the system has to be significantly increased.
U.S. Pat. No. 5,795,548 discloses an apparatus, which includes a spray drier and a particle separator upstream of the spray drier. Calcium oxide in ash obtained from the particle separator, from the bottom of the spray drier and from the last dust separator is slaked in a separate slaking apparatus. The produced material containing calcium hydroxide is then turned into a slurry, which is pumped to the spray drier and mixed in the flue gas by the sprayer.